In a DRAM as a semiconductor device, a capacitor is formed in a cylinder shape in order to obtain capacitance of the capacitor. However, recently, it has been considered to form a capacitor in a polysilicon layer. In this case, it is necessary to form a hole of a high aspect ratio in the polysilicon layer formed on a semiconductor wafer (hereinafter, simply referred to as “wafer”).
Typically, a polysilicon layer is etched by using plasma generated from a halogen-based gas such as a hydrogen bromide (HBr) gas. If a hole of a high aspect ratio is formed in the polysilicon layer, a deposit generated by etching is formed on a hard mask layer having a pattern in order to prevent the hard mask from being etched. Thus, etching selectivity of the polysilicon layer can be relatively increased. This method has been suggested by the present applicant (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Laid-open Publication No. 2010-153702
However, as depicted in FIG. 5(a), if a polysilicon layer 50 is etched by using plasma generated from a hydrogen bromide gas, a bromine radical 51 in the plasma reacts with silicon so as to produce a reactive product such as silicon bromide (SiBrx (x is a natural number)). Further, the reactive product produced by the etching includes silicon oxybromide (SiOBrx (x is a natural number)). Since, however, the etching is typically performed in a depressurized environment where an atmosphere does not exist. Thus, the generated reactive product is silicon bromide or the like without oxygen.
The generated silicon bromide is deposited on an inner surface of the hole 52 as a deposit 53 (FIG. 5(b)). As the polysilicon layer 50 is etched, the amount of the silicon bromide is increased. Since it is difficult to remove the silicon bromide by using bromine plasma or hydrogen plasma, the amount of the deposit 53 formed on the inner surface of the hole 52 is increased, and silicon is hardly exposed at the hole 52 (FIG. 5(c)). Thus, an etching rate of the polysilicon layer 50 is decreased.